The invention relates to a method for determining hydrographic parameters describing a sea swell field in situ, particularly the sea swell, the current and the water depth, by means of a radar device which provides analog signal sequences from which a sequence of digitized signals in the form of spatial coordinates is provided. From the sequence of digitized signals in spatial coordinates, a three dimensional complex value wave number frequency spectrum is determined by means of a Fourier transformation, the wave number frequency spectrum is then filtered in accordance with the principle of dispersion relationship and the wave number and frequencies of the sea swell are inter-linked for a localization of the sea swell-specific parameters by separating the signals from the noise contained in the signal sequence supplied by the radar device. Then the height of the waves is determined from the signal to noise ratio obtained and the parameters describing the current of the sea swell close to the surface and the depth of the water are determined for the three-dimensional spectral range by localization of the signal coordinates in the surface area defined by the dispersion relationship.
A radar device linked to equipment, which provides from the analog signal sequences delivered by the radar device in polar coordinates a sequence of digitized signals corresponding to the sea swell, is known from DE -OS 43 02 122.
The determination of hydrographic parameters describing in situ a sea swell field over a certain area is a theme with which the oceanographic sciences have been concerned for decades. Information concerning the behavior of an in situ sea swell field on the open sea, in coastal waters, in tide-dependent river beds and river mouth areas as well as for coastal protection measures and port construction would place the technical world including navigation, exploration and production techniques of sea-based plants in a position to develop measures for the prevention of damage from short term sea swells and the long-term behavior of sea swells. All larger nations which, as a result of their geographical location, have access to the sea or which include coastal areas exposed to sea swells, have active research programs in this field in order to receive not only short term information concerning the behavior of sea swell fields but also to obtain from the behavior information concerning long-term changes as a basis for developing measures for the protection and maintenance of the coastal land areas. It can generally be said that the sea swell and tide flows, particularly in areas close to the coast, is inhomogeneous since the water depths are different. Reference is made in this connection to current-and-depth refraction. These processes result in long term changes of the morphology. In the area of water fortifications and harbor inlets additionally a diffraction of the sea swell field occurs to which these areas are exposed and by which additional inhomogeneities of the sea swell are induced.
Mechanical and optical in situ current sensors determine a value of a current, which is representative for a small measuring volume with typical dimensions of 10 cmxc3x9710 cm (point measurement). Vertical current profiles can be established by ADCPs (Acoustic Doppler Circuit Profiler). Horizontal current profiles, that is, current maps can be calculated so far from measurements obtained by HF radar devices. However, the application field of these remote exploration sensors is limited to salt water. Areas of up to 20 kmxc3x9720 km can be measured in this manner, however, with a spatial resolution of the current map of only 500 mxc3x97500 m.
From the image sequences of nautical radar using local analysis procedures current maps can be provided with a spatial resolution which is improved by an order of magnitude and the procedure can also be used in sweet waters. Because of the high spatial resolution also small-scale inhomogeneities of the current field such as whirls can be measured. The area that can be maintained is generally 2 kmxc3x972 km.
Depth maps can be established in coastal waters by echo sounding. This procedure however is time-consuming and expensive (vessel times). Echo sounding can therefore be performed only sporadically. From the image sequences of nautical radar, however, depth maps can be prepared continuously at relatively low financial and logistic expenses by a local analysis procedure. On an experimental basis, algorithms have already been developed which permit the preparation of a map of the water depth based on certain hydrographic conditions with the knowledge of the surface currents by the analysis of the radar image sequences of inhomogeneous water surfaces. A method developed by Bell [1998] requires however that the wave field consists locally of a single wave wherein the wavelength and direction of movement is spatially variable as a result of the variable water depth. Hessner et al. [1999] was the first to divide the wave field on the basis of wave frequency before determining from the individual frequency components pixel by pixel the water depth on the basis of the dispersion relationship. This method can be used if the directional dispersion of the sea swell state to be analyzed is low, since, otherwise, partial waves arriving from different directions result in interferences.
Another procedure which is utilized for the determination of parameters which describe an in situ sea swell field resides in the measurement of a one-dimensional frequency spectrum and possibly also of the moments of directional distribution of the sea swell at individual locations by means of the so-called sea buoys. Sea buoys however are not suitable for use in low depth waters, particularly in the surf or breaker range and they permit essentially only a point determination of the sea swell field. A very important disadvantage of the known method for determining the hydrographic parameters of a sea swell field by way of sea buoys is the insufficient directional characterization of the sea swell field.
Another method is the so-called global radar image sequence analysis. With the global analysis procedure values of hydrographic parameters are determined, which represent the whole analysis area. The method is used for homogenous sea swell fields, that is, sea swell fields in which the hydrographic parameters are spatially constant over the whole area of analysis.
The signal sequences (radar image sequences), interpolated onto a Cartesian grid, are converted by a three-dimensional fast Fourier transformation (3D FFT) to a three-dimensional complex-value frequency-wave number spectrum. By the global sequence analysis, the variance spectrum calculated by the formation of the square of the absolute value is evaluated.
Subsequently, the water depth d and the components of the horizontal current vector ux and uy are determined by an adaptation of the sea swell signal coordinates of the image spectrum as selected with a threshold value of the variance to the theoretical dispersion relation of the sea swell waves [Senet, 1996, Outzen, 1998]. The method used for calculating the water depth and current is preferably the so-called xe2x80x9cLeast-Squares-procedurexe2x80x9d. The current and depth values obtained by this procedure, which are representative for the whole analysis area, are the base values for the global procedure.
The dispersion relation defines an area in the spectral space, called xe2x80x9cdispersion envelopexe2x80x9d, whose shape depends on the value of the current and the water depth. The localization of the sea swell signal on the dispersion envelope makes it possible to utilize the dispersion envelope after the calculation of the current and the water depth as a spectral filter for the separation of the signal and the noise components of the image spectrum.
The sea swell spectrum, that is, the variance spectrum of the surface deviation, is linked linearly, by way of an image transmission function, to the signal to noise ratio of the image spectrum, that is, the gray value variance spectrum. The image transmission function can be parameterized by an exponential function with the value of the wave number as basis. The significant wave height is proportional to the square root of the signal-to-noise ratio of the image spectrum (Niete et al., 1999). The calibration parameters are determined at the beginning of a measuring phase by comparison with an in situ sensor for the wave height, that is, the sea swell buoy already mentioned. After the calibration phase the image-sequence analysis can be performed independently of further in situ sea swell measurements (Ziemer, 1995). Further base output values of the global method are a 180xc2x0 oriented sea swell spectrum and a value of the significant wave height representing the whole analysis area.
Also, the global image sequence analysis according to the so-called WaMoS-process described earlier has not provided satisfactory results so far, since the three-dimensional variance spectrum, on which this process is based, does not permit a complete description of the space-time correlation of the sea swell field if the sea swell field is inhomogeneous. Rather, this procedure provides only global parameters weighted over the measuring parameters and is therefore sufficient only for the open sea where a homogeneity of the sea swell field can be assumed because of sufficiently large water depths.
In addition to the procedures mentioned above for determining a depth map an analysis of inhomogeneous sea swell fields or, respectively, inhomogeneous water surfaces has been tried on the basis of a computation method which is known to the experts as MUSIC (Multiple Signal Classification). However, this procedure has been rejected by the experts since the duration of a MUSIC-based analysis of a sea swell field does not permit an operative analysis as the procedure is too slow.
It is therefore the object the present invention to provide a method for determining hydrographic parameters which describe a sea swell field in situ using a radar device wherein first analog signal sequences are provided by the radar device which exactly describe the behavior of the sea swell fields on the open sea and also near the coast. The method should also make a continuous recording of the sea swell field possible in order to provide a decision basis for the need of measures for the protection of a coast line based on the parameters collected and also to obtain indications concerning the effectiveness of measures established earlier and with respect to the influence of diffraction by the sea swell field, which may be caused by water control installations.
Furthermore, the generation of highly accurate horizontal current maps should be facilitated and influences on the behavior of the sea swell field by navigation, exploration and transport equipment should be explored. The spatial distribution of the hydrographic parameters should also be possible by means of the method according to the invention essentially on a real time basis, that is, within a period during which the values of the parameters cannot significantly change.
In a method of determining hydrographic parameters describing a sea swell field from analog signal sequences supplied by radar devices, wherein a sequence of digitized signal in spatial coordinates is generated from the analog signal sequences and, by Fourier transformation, a three-dimensional complex value frequency wave number spectrum is determined therefrom, which is filtered on the basis of the dispersion relation principle that inter-links the wave numbers and the frequencies of the sea swell for a localization of the sea swell-specific parameters by separating the signals from the noise and determining the signal to noise ratio and from the ratio the height of the waves and, by localizing the signal coordinates in the surface area defined by the dispersion relationship, parameters describing the surface currents of the sea swell field in a three-dimensional spectral space and the water depth, from the phase information concerning the waves monitored in the sea swell field the parameters of the sea swell field are determined.
The method according to the invention utilizes the effect of the wind on the sea swell as observed on the sea surface in a sea swell field by a radar device. The small-scale roughness of the sea surface as generated by a local wind field observed in the sea swell field results in a radar stray reflection, which is modulated by the waves in the observed sea swell field. The sea swell is therefore depicted by the radar device which, in principle, may be a common nautical radar device, as soon as the wind speed exceeds a predetermined threshold value, typically 2 to 3 msxe2x88x921 and the waves are large enough, for example  greater than 40 m so that they can be resolved by the radar device.
The advantage of the method according to the invention resides essentially in the fact that it is possible with this method to determine, essentially in real time, the spatial distribution of hydrographic parameters of an inhomogeneous sea swell field that is the space time correlation of the wave field observed, the local sea swell spectrum with complete directional resolution, the field of currents near the water surface and the water depth. From these parameters, maps can be prepared concerning the water surface currents, the water depth and the local wavelength and direction distribution of the energy at the location of the sea swell field observed, so that excellent navigation aids can also be provided for the ship traffic.
With the results of the method, a continuous surveillance of the bathymetry can be achieved which causes variations in the tide currents in the coastal waters. Also, based on a possible continuous determination of the hydrographic parameters of the observed sea swell field also measures for improving the coast line protection, for example, by sand deposits, can be initiated in order to avoid the loss of land. Also, the effects of the measures introduced on the basis of the established parameters can be continuously monitored as to their effectiveness.
Because of large water depths, sea swell fields in the open sea are generally homogenous with respect to the wave number K, the frequency xcfx89, the wavelength xcex and the period xcfx84. Sea swell fields on the open sea are therefore generally called homogenous sea swell fields or homogenous sea swells. In order to be able to use the method according to the invention also for the analysis of so-called inhomogeneous water surfaces, particularly also the inhomogenous water surfaces as they are present in low depth coastal waters, method steps of the global analyzing procedure are adapted and special procedures for the local analysis are developed such that the phase information of the waves of the sea swell contained in the complex-value frequency wave spectrum is used for the determination of the parameters in an inhomogeneous sea swell field.
While, during the analysis of homogenous sea swell fields as they have been explained above, the hydrographic parameters are determined by an analysis of the variance spectrum, with the method according to the invention for a local analysis of radar image sequences of the sea swell, additionally the phase information is employed which contains the information concerning the local image structure. It is assumed in this connection, that the wave field consists locally of individual complex-value sine waves. This condition is ensured after the frequency and direction resolution of the depicted wave field ahead of the local analysis by the dispersion relation of the linear sea swell as it is assumed for an inhomogeneous sea swell field on a local spatial scale. With a fixed frequency and wave direction of a partial wave, maximally two values of the wave number fulfill the dispersion relationship. With an observation of the sea swell field by means of the radar device from a fixed location only the smaller of the two solutions is actually relevant. The complex-value three-dimensional frequency wave signal spectrum (image spectrum), which is present already resolved by means of the Fournier transformation into individual frequency support places, is spectrum-filtered by a directional filter and a filter defined by the dispersion relationship.
Subsequently, the selected spectral intervals are reverse transformed by a two-dimensional Fourier transformation into the local frequency range.
The invention will be described in greater detail on the basis of an example with reference to the accompanying drawings.